Fishing - good for the environment?

Being a Dog owner / walker and long time resident of Bottesford I enjoy being able to walk down the Beck with the wife and dogs, we get to see all kinds of wildlife, The Rabbits and Hares, the Foxes and the Deer, the Birds especially the Kingfishers, the Fish and all manner of other critters that we come across.

Its whilst pursuing my walks along the Beck that I have over the recent months on several occasions come across a couple of shall we say Eastern Europeans fishing the Beck, nothing too odd you might say but these gentlemen were not sat in one spot trying their luck as per your sport angler, but more just cruising up and down the banks catching the fish and popping them in a net which unlike the regular angler was not in the water, one can only assume that these were for consumption at a later date.

Speaking with fellow dog walkers they said they have also seen them and one stated that they have been chased off the old Ashby sewer works pond for taking the fish out of that, a place popular with sports anglers and stocked with on all accounts some quite large fish, another fellow walker asked me if I had seen the group of shall we say non Europeans in the Beck the other day, he went on to tell me how he came across two youngish chaps in the water near the Baldwin Ave area, he thought it odd but it was a warm night and carried on walking as he made his way into the next field he came across two more chaps stood in the water with a net across the Beck catching the fish and putting them in a Polystyrene box which he said had several large specimens already in it.

It raises several questions in my mind, Did they / do you need a licence to fish on the Beck? is it legal to take these fish (not sure on breed, Rudd/Tench?) home? Is the netting of fish legal? Do we have wardens to monitor said activity? Would it be safe to challenge said gentlemen?